The present invention relates generally to logging-while-drilling tools in the borehole logging industry. More particularly, the invention relates to sonic logging-while-drilling receivers and their configuration in a drill collar.
Research and development in the field of sonic logging-while-drilling has resulted in the transverse mounting of an acoustic source in a drill collar. The details of a transversely mounted source are disclosed in copending U.S. Pat. application Ser. No. 288,742 entitled, "Method and Apparatus for Performing Acoustic Investigations", invented by Kostek et al., and assigned to Schlumberger Technology Corporation. The present application is a continuation of that application, the details of which are incorporated by reference herein.
The acoustic source of a traditional sonic wireline logging tool is mounted along a vertical central axis in the length of the tool and transmits acoustic energy at equal levels in all directions about the vertical central axis. These signals are typically transmitted through holes in the tool housing and into the formation. The traditional sonic wireline logging tool also includes a set of receivers mounted in a vertical line through the central axis of the tool housing positioned at a particular distance from the source. Such a tool is disclosed in U.S. Pat. No. 3,191,141 to Schuster, and assigned to Schlumberger Well Surveying Corporation.
The use of a traditional source in a sonic logging-while-drilling tool is not feasible because holes cannot be cut into the drill collar itself to allow the acoustic energy transmitted from the source to pass through the drill collar. There are two principle reasons for this. The first reason is that holes in the drill collar greatly reduce its mechanical integrity resulting in destruction or loss of effectiveness in the harsh drilling environment. The second reason is that mud is circulated through the drill collar to lubricate the drill bit and to allow the cuttings to flow up the collar for removal. If holes were made in the drill collar, it would disrupt or eliminate the required flow of mud. To accommodate these concerns, a transversely mounted source has been developed such that only the ends of the source are exposed through a pair of holes in opposite sides of the drill collar. A problem arises in that the signals produced by the source are not exclusively monopole in nature like those produced by a traditional sonic logging tool source. Instead, in addition to the monopole excitation mode of the acoustic signal, the transversely mounted source produces a series of excitation modes including a quadrupole, an octopole, etc which are symmetric with respect to an axial plane perpendicular to a central axis through the length of the source. Each of these excitation modes is transmitted into the formation and is recorded by the receivers.
It is desirable to process the recorded signals using traditional techniques. Such a technique is known from U.S. Pat. No. 4,594,691 to Kimball et al., and assigned to Schlumberger Technology Corporation. Traditional signal processing generally concentrates on the monopole excitation mode of the acoustic signals. Therefore, it is essential to reduce or eliminate the effects of the other modes which may interfere with the monopole mode. A receiver with the ability to reduce or eliminate the effects of the modes including the quadrupole, octopole, etc. is necessary. Such a receiver accentuates the monopole component which is essential for traditional processing techniques.